iOS HTML checkbox screen blocked if keyboard is shown

I have an iOS application which views URL inside UIWebView. There is a form in this URL with two text input fields and one checkbox.

When the cursor is inside one of the text input fields and the keyboard is shown, then I check the checkbox, the screen becomes semi-black and the form dialog is removed. But when I check the checkbox normally - without the keyboard showing -, it works fine and no error happens.

Here is the form code:

<form action="<?php echo site_url('account/login');?>" class="form-horizontal" role="form" method="post">
                        <!-- Form Group -->
                        <div class="form-group">
                            <!-- Label -->
                            <label for="user" class="col-sm-3 control-label"><?php echo lang_key('email'); ?></label>
                            <div class="col-sm-9">
                                <!-- Input -->
                                <input type="text" class="form-control" name="useremail" placeholder="<?php echo lang_key('email'); ?>">
                            </div>
                        </div>
                        <div class="form-group">
                            <label for="password" class="col-sm-3 control-label"><?php echo lang_key('password'); ?></label>
                            <div class="col-sm-9">
                                <input type="password" class="form-control"  name="password" placeholder="<?php echo lang_key('password'); ?>">
                            </div>
                        </div>
                        <div class="form-group">
                            <div class="col-sm-offset-3 col-sm-9">
                                <input type="checkbox" name="remember"> <?php echo lang_key('remember_me'); ?>
                            </div>
                        </div>
                        <div class="form-group">
                            <div class="col-sm-offset-3 col-sm-9">
                                <!-- Button -->
                                <button type="submit" class="btn btn-red"><?php echo lang_key('login'); ?></button>&nbsp;
                                <button type="submit" formaction="<?php echo site_url('account/signup');?>" class="btn btn-white"><?php echo lang_key('sign_up'); ?></button>
                            </div>
                        </div>
                    </form>

The 'remember_me' checkbox is the one which causes the issue if the keyboard is shown when it is checked